In the chloride process for making titanium dioxide, titanium tetrachloride is oxidized in the vapour phase to titanium dioxide. The titanium dioxide and other reaction products are then carried forward through externally cooled conduits capable of heat exchange, where they are cooled and coalesced. During the cooling process, the oxide can be accreted upon the walls of the reactor and on other surfaces in the reaction zone. Accretion of TiO2 product usually results in a number of deleterious effects, such as loss of product quality due to excessive retention of the product in a high temperature environment, drop-off of wall accretion into the main product stream, localized overheating of equipment due to poor heat transfer through the accretion and plugging of gas entries.
These problems resulting from the formation of TiO2 particulate deposits on the internal walls of conduits can be reduced by introducing relatively hard granular abrasion particles into the hot suspension. Silica sand is preferably used, as granular scrubs for removing TiO2 deposits from the internal surfaces of cooling conduits containing a hot suspension of TiO2 particles, for example, suspension containing from 0.1% to 0.5% by weight of titanium dioxide pigment. The specific particles for such scrubbing can have average diameters in the range of size distribution of from about 1 mm to about 0.5 mm.
For simplicity of description, the above-described scouring particles are herein under referred to as “scrubbing solids” or simply “scrubs.”
The scrubs are introduced before the reaction stream exits the reactor into cooling ducts to scrub and remove build-up from the interior of the flue pipes downstream from the TiO2 oxidation section. The reaction stream with the scrub solids is cooled.
The pigment and scrub solids are discharged from the cooler to a cyclone and then a bag filter to separate the gases. The solids are then slurried in water.
Since such scrub particles constitute undesired grits in the pigment product, it is necessary to remove them.
The slurry is screened for subsequent treatment and silica sand scrubs are separated from the titanium dioxide slurry as they can contaminate the final titanium dioxide pigment product.
As a matter of fact, the silica sand scrubs reclaimed from a titanium dioxide slurry resulting from a chloride manufacturing process presents environmental problems as there is no means for complete reprocessing and recycling of the scrub material. Previously, no significant technically and economically practical use of this waste has been provided so the waste has either been collected and used as landfill or moved to a dump. As the sand material becomes available in large quantities as a waste material at site, it becomes increasingly more environmentally invasive and its disposal becomes more and more difficult and expensive.
It is well recognized in this industry that any use to which these silica sand scrub wastes can be put would be highly beneficial. Therefore, there exists a need for a process for converting the silicon dioxide content of this waste into products such as silicates and silica gel, or precipitated silicas, which represent high grade value-added products exhibiting a wide range of useful applications.
There are a number of process techniques for the production of water soluble alkali silicates. These include, inter alia, either the dry process or the wet process. In the dry process, quartz sand is used as source of silicon dioxide and is caused to react with soda or sodium carbonate in melt temperatures in the range from 1400° C. to 1500° C. and subsequently dissolved in water under pressure at elevated temperature in another processing step.
In the wet process, quartz-sand is made to react in a pressure reactor at a temperature of 180° C. to 250° C., under saturated steam pressure corresponding to the temperature used, with an aqueous alkali-hydroxide solution.
Thus, the source of silica in either of these two above processes is quartz sand. In the case of wet process, the conversion reaction is sluggish and does not take place quantitatively and results in low modulus sodium silicate. If sodium silicate with a high SiO2:Na2O molar ratio is to be produced, the essential prerequisite is the selective utilisation of higher soluble silica modifications like amorphous silicon dioxide, such as that from industrial flue dusts, from naturally occurring amorphous silicon dioxide containing materials and cristobalite modification of the silica sand, either available from nature or prepared by tempering process, as source of silica in the process. Alongside these, industrial by-and-waste products play as alternative raw materials for the production of cheap sodium silicate solutions.
As described above, the dry process has the disadvantages that the process is highly expensive both in terms of investment and maintenance costs and in terms of energy consumption and, further, it causes considerable air pollution. It is characterized, in addition, by a particularly careful selection of the silicon dioxide material, especially with a view toward the content of iron and aluminum oxides.
The following prior art processes involve conventional processing techniques using different sources of silicon dioxide for the production of sodium silicate solutions.
U.S. Pat. No. 4,190,632 teaches a process for producing sodium silicate solution by treating air-borne dust containing silicon dioxide with an alkali to form an alkali metal silicate solution at a temperature of about 60° C. to about 110° C., wherein the air-borne dust is a waste product that has been removed from the flue gases originating from silicon metal or ferro silicon alloy production processes with particle size below 90 micron, followed by purification by adding activated charcoal and/or oxidation agents such as sodium peroxide or hydrogen peroxide and filtration. The purifying agents are added 1 hour before the end of boiling. The flue dusts used as starting materials has high amorphous silica, SiO2, content of 89.5% to 97.5% by weight, the remaining consisting of impurities. The sodium silicate solution obtained had a SiO2:Na2O molar ratio of 4.1:1. The process is characterized in that very fine amorphous siliceous particles in the form of flue dust are used as source of silicon dioxide and that the product is subjected to purification steps involving activated charcoal and oxidizing agents.
U.S. Pat. No. 5,000,933 discloses a process for direct hydrothermal production of high purity sodium silicate solutions having a high SiO2:Na2O molar ratio by reaction of a silicon dioxide source with aqueous sodium hydroxide solutions, or with aqueous sodium silicate solutions having a lower SiO2:Na2O molar ratio, characterized in that the silicon dioxide source provided contains a sufficient fraction of natural cristobalite phase, or synthetic cristobalite produced by conditioning other crystalline forms of silicon dioxide by heating at a temperature from about 1200° C. to about 1700° C. in the presence of a catalyst but below the melting point of silica, before the hydrothermal treatment. Even with use of readily soluble natural or synthetic cristobalite containing silicon dioxide, the hydrothermal reaction has to be carried out in a closed pressure reactor at a temperature in the range of from 200° C. to 250° C., for example, 225° C., under saturated steam pressures corresponding to the temperature, in order to get a SiO2:Na2O molar ratio of from 3.3 to 3.5:1.
It can be seen that the prior art processes described above follow methods for making sodium silicate solutions characterized in that the raw material comes as specialized product such as natural or synthetic cristobalites and in that very fine by-product and waste-product silicon dioxide sources are utilised. It is specific in that the cristobalite modification of the silica sand is used either exclusively in a single stage reaction process or by preparing a cristobalite modification from natural sand through a tempering process followed by a two-stage reaction process. This means a high process cost due to the high price of the raw material containing cristobalite. The tempering process option involves both a highest investment and a high process risk. A single stage reaction process using cristobalite might exhibit the lowest investment effort but exhibits the highest operational costs due to the need for cristobalite as a raw material.
There exists a need for a reprocessing inexpensive silica sand scrub wastes emanating from titanium dioxide production processes using chloride technology, as a low cost starting material. There is a need to convert this waste into high modulus sodium silicates and silicas, which represent value-added products exhibiting a wide range of applications, especially for captive use for silica coating of titanium dioxide pigment and as fillers and extenders in diverse industrial products.